There are numerous laboratory systems and medical as well as pharmaceutical devices which require precise pipetting operations in order to obtain satisfactory analytical precisions. For this purpose, the level in test tubes, titre plates and other liquid containers must be determined precisely. There are also applications in which it is a question of detecting foam-liquid phase boundaries. Hereinafter the term of the phase boundary is used both for transitions between gaseous and liquid media (gas-liquid phase boundary) and also for transitions between various liquid media (foam-liquid phase boundary).
Such a determination of the phase boundary is particularly important when it is a question of the automation of measurement or experimental sequences. The so-called level determination is typically made by means of a detection of the liquid level, i.e. the position of the phase boundary between air and liquid is determined. This process is also designated as “Liquid Level Detection” (LLD).
In the prior art, various methods for level determination are known, which are based on different physical principles such as, for example, detection of the light reflected from the surface of the liquid or measurement of electrical properties of pipettes when they are brought in contact with the liquid. Since a gas and a liquid have significantly different dielectric constants, the gas-liquid phase boundary can also be determined by means of a capacitance change.
Liquid Level Detection is used, for example, in pipetting devices. Here when sucking in with a pipette, the pipetting needle should be immersed as little as possible into the liquid to be pipetted in order to keep any contamination with sample liquid as low as possible. During sucking in, the pipetting needle is therefore typically immersed only a few millimeters below the liquid level. However, it must be ensured that the pipetting needle is immersed sufficiently far and therefore no air can be sucked in. During the sucking in process, the pipetting is then continuously tracked to the decreasing liquid level so that it always remains immersed to the same depth in relation to the liquid level. After the sucking-in, it can then be calculated, e.g. by means of the sucked-in volume and the cross-sectional area of the liquid container, at which height the gas-liquid phase boundary should be located. During emergence of the pipetting tip, an emergence signal can then be compared with the calculated position of the gas-liquid phase boundary in order to thus verify the pipetting process.
On the one hand, it is therefore desirable to position the pipetting tip just below the liquid surface, on the other hand the level can vary substantially from one liquid container to another, which is why the pipetting tip must be positionable precisely. For this it is extremely important to be able to correctly detect the liquid surface.
In some cases however, the reliability of the detection of the liquid surface with the known methods is not satisfactory, in particular in the case of liquids which are liable to foam formation.
Therefore it is important to be able to distinguish between a foam and/or liquid contacting of a sensor which can be delivered in a liquid container.